Background: The introduction of solid foods is an important dietary event during infancy and is associated with a time of dramatic shifts in gut microbial composition. The influence of solid food introduction on gut bacterial dynamics remains understudied. Methods: 15 healthy, full-term, vaginally born, and breast-fed infants of the Baby, Food and Mi sub-study of the Baby & Mi Study were investigated. Caregivers were asked to collect daily stool samples and food diaries for 17 days, commencing three days prior to the introduction of solids. Additional stool samples were available up to one year as part of the Baby and Mi study. The exposure of interest, nutritional patterns, was analyzed using food composition output from ESHA’s Food Processor. The number of food items and food groups introduced were used to calculate dietary diversity scores. The outcome of interest, gut bacterial dynamics, was analyzed using RStudio. Results: The mean (SD) age at the introduction of solid foods is 5.5 (0.66) months (n = 15). Over the study period, the proportion of estimated energy intake from solid foods was low (7.5%; SD 6.74%) (n = 14). Alpha diversity increased over time and was highest at 1 year. The gut microbial community influenced by dominant bacterial taxa changed with increasing age. With introduction of solids, individual community composition changed, though to a varying extent. Shannon alpha diversity was directly associated with calories from carbohydrates, particularly daily fiber intake. The infant’s dietary diversity score was directly associated with alpha diversity and was also positively associated with the degree of change occurring in this time period. Conclusion: Fiber intake and the dietary diversity scores had the closest relationships to the gut microbiome’s alpha diversity and community structure in infants at the time of solid food introduction. / Thesis / Master of Science (MSc) / The introduction of solid foods is an important life-event during infancy. This is also when the gut microbiome is developing to its mature state. Since nutrition is an important factor influencing the microbiome, investigating the dietary choices at the introduction to solid foods is the aim of the following study. Here, daily stool samples and food diary entries were collected for 15 healthy, breast-fed infants. It is important to measure the diversity of the bacteria in the gut of an individual (alpha) and between people (beta), as well as bacteria present. Carbohydrates drive the change in alpha diversity, especially fiber. Feeding infants a diet with many different foods shows increased alpha diversity and change in the microbiome immediately after introduction. Interestingly, the infant gut microbiome reacts to fiber in a manner comparable to the adult gut microbiome, i.e. increased bacterial diversity, which is associated with better health outcomes in adults.

Early life factors play important roles in disease susceptibility in later life. However, the relationship between dietary factors in early life on childhood growth, especially linear growth, remains unclear. This research aimed to improve our understanding of the associations between dietary factors in early life (i.e., infant feeding practices and age of introduction of solid foods) and childhood growth, especially using ulnar length as a surrogate measure of length/height, in a cross-sectional study of 1634 mother-child dyads across eight study centers in the National Children's Study Formative Research in Anthropometry in the United States from 2011-2012 (Chapter 1). Chapter 2 described the data acquisition and preprocessing procedures used in this research and provide practical guidelines of data quality control. In Chapter 3, predictive models for exclusive breastfeeding (XBR) initiation and duration was developed. Discriminant analysis revealed maternal sociodemographic factors had greater discriminating abilities to predict XBR initiation and XBR for 6 months, compared to child birth characteristics and maternal perinatal factors. Chapter 4 demonstrated that ulnar length can serve as an accurate and reliable surrogate measure of recumbent length in healthy infants/children aged 0-1.9 years and of height in healthy children aged 2-5.9 years, respectively. Bland-Altman plots and mixed-effects linear regression analyses showed that the three simple and portable tools (i.e., caliper, ruler, and grid) used to measure ulnar length could be used interchangeably in terms of prediction accuracy. Chapter 5 focused on assessing the interplay among gestational weight gain (GWG), birthweight, infant feeding practices, and childhood anthropometrics. Longer duration of breastfeeding reduced the positive associations of GWG and birthweight with weight-for-age z-scores, weight-for-height/length z-scores, and body mass index-for age z-scores in non-Hispanic Whites. These findings underscore the importance of promoting breastfeeding among women with excessive GWG to mitigate childhood obesity. Longer breastfeeding and a later age at introduction of solid foods had positive effects on ulnar length, a linear growth parameter of upper extremity, in Hispanics. Future prospective research aiming to investigate the underlying mechanisms that drive ethnic variation in these associations between early life dietary factors and childhood growth is warranted (Chapter 6). / text